1. Field of the Invention
The present invention relates to an image forming apparatus using an electrostatic recording method, an electrophotographic recording method, or the like.
2. Description of the Related Art
As an image forming apparatus using an electrostatic recording method, an electrophotographic recording method, or the like, conventionally, an electrophotographic printer as illustrated in FIG. 8 has been used. Referring to FIG. 8, with respect to this conventional image forming apparatus, a basic configuration, a function, and the like will be described below.
First, the image forming apparatus includes a plurality of image forming units 10 each configured to form a latent image on a photosensitive drum 11 as an image bearing member using light, magnetism, electric charge, or the like, develop the latent image, and obtain a visible image; an intermediate transfer member 30 located above the image forming units 10 and configured to successively receive the visible image from each image forming unit 10 to form a multi-color image; a transfer unit 35 configured to transfer the multi-color image on the intermediate transfer member 30 to a recording material P; a paper feed unit 20 configured to convey the recording material P from a cassette to the transfer unit 35; and a fixing unit 40 configured to fix the multi-color image transferred on the recording material P in the transfer unit 35 to the recording material P.
As the fixing unit 40, a fixing unit of the film heating type for heating via a fixing film small in heat capacity is adopted as an on-demand method, where a heat transfer efficiency is high and the start-up of an apparatus is quick. This fixing unit of the film heating type will be described referring to FIG. 9. In the fixing unit 40, a fixing nip N is formed with a pressure roller 41 for applying the predetermined pressure and a heating unit 42. The heating unit 42 includes a film 43, a film guide 44, a heater 45, and a thermistor 46.
In the fixing unit of the above-described on-demand method, when a recording material small in size in a direction perpendicular to a paper pass direction (hereinafter, referred to as paper width direction) is fed, there has been a problem in which heat is left in a paper non-pass portion, through which the recording material does not pass, in the fixing nip N to increase the temperature of the paper non-pass portion. In a case where the temperature of the paper non-pass portion has become extremely high due to the passage of the small size recording material, when a large size recording material is fed directly after the small size recording material, since the temperature of the paper non-pass portion for the small size recording material is too high, a toner on the recording material may be deprived by a fixing film in a portion of the large size recording material corresponding to the paper non-pass portion for the small size recording material, so that an image defect such as hot offset may easily occur. Further, the temperature of the paper non-pass portion may become higher than an assumed temperature, thus causing a failure.
To address such an issue, a configuration is discussed in which in the above-described fixing unit, a plurality of thermistors disposed on the back of a heater is provided. More specifically, a center thermistor located at the center in a recording material width direction orthogonal to a recording material conveyance direction and an end thermistor located in an area corresponding to the paper non-pass portion for the small size recording material are disposed. In a case where the small size recording material is fed, the throughput of the recording material is decreased depending on the detected temperature of the end thermistor (Japanese Patent Application Laid-Open No. 2002-91226).
FIG. 10 illustrates the position of a center thermistor and an end thermistor in a paper width direction in a fixing nip portion, and a temperature distribution when a large size recording material and a small size recording material are fed. Temperature T0 is a limit temperature without a harmful effect due to an image defect such as the above-described hot offset or melt. Temperature T1 is a threshold value when the large size recording material is fed. Temperature T2 is a threshold value when the small size recording material is fed. When the temperature of the end thermistor while the paper is fed reaches the respective threshold values, the decrease of throughput is performed so as to prevent the temperature from increasing beyond the respective threshold values. As illustrated in FIG. 10, a peak (maximum temperature) in a paper width direction of the small size recording material is located closer to the center away from the end thermistor position than a peak of the large size recording material. Thus, the threshold temperature of the end thermistor when the small size recording material is selected is set lower than that when the large size recording material is selected. This threshold temperature is set depending on the selection of the recording material size by a user. In other words, when the user selects the recording material size via a personal computer or the like, a printer engine unit automatically sets the optimal threshold temperature according to size information selected by the user.
Accordingly, when the small size recording material is selected, the threshold temperature is set to the temperature for the small size recording material. When the detected temperature of the end thermistor reaches the threshold temperature for the small size recording material, the operation is shifted to the decrease of throughput of the recording material to reduce temperature rising at the end portion of the fixing unit. However, in the above-described conventional image forming apparatus, when a user erroneously sets the small size recording material to the large size recording material, a reduction in temperature rising at the end portion may become difficult. For example, a case is assumed in which the user selects A3 size paper, though the user should select A4 size paper, via a personal computer with A4 size paper set to a manual paper feed unit, which is unable to recognize the paper size.
The manual paper feed unit cannot recognize the paper size. However, since A4 size paper and A3 size paper are different in width and length, the length of the paper can be detected by a paper feed sensor that is normally provided on a printer. Thus, the printer engine unit can recognize that the actually conveyed paper is not A3 size paper selected by the user but is A4 size paper. In this case, paper conveyance may be discontinued to inform the image forming apparatus of a selection error.
However, in a case where in the size of the paper set on the manual paper feed unit, the width is equivalent to A4 size (or a further smaller size) and the length is equivalent to A3 size (in the case of nonstandard-size paper), when the user selects A3 size paper, even if the paper feed sensor detects the length of the paper, the length is equivalent to A3. Thus, the printer engine unit recognizes the actually conveyed paper is A3 size paper. In this case, the threshold temperature is set to the temperature for A3 size paper. Though the small size recording material is actually fed, the decrease of throughput is not performed until the detected temperature of the end thermistor reaches the threshold value for the large size recording material. Accordingly, the temperature peak value in the paper non-pass area for the small size paper is increased to exceed temperature T0 (FIG. 11). As the result of this, a harmful effect, such as an image defect such as the hot offset and a paper pass failure due to thermal deformation of a component, occurs.
On the other hand, if an exclusive sensor configured to detect the paper width is provided on the paper conveyance path, the above-described issue may be addressed. However, the exclusive sensor may cause a cost increase.